runtime: ensure fp is 0 for topofstack() functions
runtime: ensure fp is 0 for topofstack() functions

When functions are dispatched by runtime.gogo, ensure fp is 0 if the
function is topofstack() in Go terms.

4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go